The invention relates to the growing of crystals. More particularly, the invention is directed to a method of orienting seed crystals and a method and apparatus for cooling the seed crystal during the crystal growth operation.
It is known in the art of growing single crystals to place a seed crystal in the bottom of a crystal growth vessel and to charge the material from which the crystal is to be grown into the vessel thus covering the seed crystal. The crystal growth material and a portion of the seed crystal are melted and allowed to cool, resulting in the nucleation of the growth material on the seed. It is of critical importance that only a portion of the seed crystal is melted with the growth material.
It is taught in U.S. Pat. No. 4,096,025 that a seed crystal can be oriented according to its growth planes in order to reduce thermal strain generated in the growth material by the heating and subsequent cooling cycles. Additionally, several methods for controlling the temperature of the seed crystal during crystal formation and thus the portion of the crystal which remains solid have been suggested. U.S. Pat. Nos. 3,653,432 and 3,898,051 both suggest active heat exchange systems which circulate an inert gas about the growth vessel in order to effect temperature control. U.S. Pat. No. 4,015,657 discloses an apparatus which uses a refrigerant to control the seed crystal temperature gradient. Finally, U.S. Pat. No. 4,264,406 provides a vessel support base with a circulating cooling fluid therein and a movable vessel shroud disposed between the vessel and the furnace with a circulating cooling fluid contained therein also.
The prior art temperature control systems are encumbered with certain liabilities which are eliminated by the present invention. For example, the use of circulating cooling fluid dictates the need for pumps, conduits and seals able to withstand the severe thermal conditions of the crystal growth process. The complexity and expense are self-evident in such prior art systems. Moreover, such systems often require a person to watch the seed melting and to physically adjust the seed crystal position in relation to the temperature profile of the furnace.
It is an object of this invention to provide a crystal growth process which is significantly simplified so that reliable blind seeding is possible.
It is also an object of this invention to provide a passive seed crystal temperature control method and apparatus.
It is still another object of this invention to provide a seed crystal orientation process which facilitates the production of crystal devices and greatly simplifies the manufacture of these devices.